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Date:   Fri, 8 Jan 2021 21:21:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, juri.lelli@...hat.com,
        valentin.schneider@....com, qais.yousef@....com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        tim.c.chen@...ux.intel.com, benbjiang@...il.com
Subject: Re: [RFC][PATCH 1/5] sched/fair: Fix select_idle_cpu()s cost
 accounting

On Fri, Jan 08, 2021 at 10:27:38AM +0000, Mel Gorman wrote:

> 1. avg_scan_cost is now based on the average scan cost of a rq but
>    avg_idle is still scaled to the domain size. This is a bit problematic
>    because it's comparing scan cost of a single rq with the estimated
>    average idle time of a domain. As a result, the scan depth can be much
>    larger than it was before the patch and led to some regressions.

> @@ -6164,25 +6164,25 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  		 */
>  		avg_idle = this_rq()->avg_idle / 512;
>  		avg_cost = this_sd->avg_scan_cost + 1;
> -
> -		span_avg = sd->span_weight * avg_idle;
> -		if (span_avg > 4*avg_cost)
> -			nr = div_u64(span_avg, avg_cost);
> -		else
> +		nr = div_u64(avg_idle, avg_cost);
> +		if (nr < 4)
>  			nr = 4;

Oooh, could it be I simply didn't remember how that code was supposed to
work and should kick my (much) younger self for not writing a comment?

Consider:

       span_weight * avg_idle               avg_cost
  nr = ---------------------- = avg_idle / ----------
               avg_cost                    span_weigt

Where: avg_cost / span_weight ~= cost-per-rq


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